1. Field of the Invention
The present invention is directed to uninterruptible power supplies and more particularly to a high speed transfer switch operable to transfer alternating current to a load from a storage source of direct current upon the failure of the normal alternating current power line feeding such load.
2. Description of the Prior Art
In today's wide use of computers, data processors, controllers, etc. in data processing systems, it is extremely critical that the system be supplied with continuous power during its operation. Due to inadequate capacity and increasing load demands, commercially-supplied power is often subject to complete failure of the power signals or a reduction in the magnitude of the available voltage during peak demand periods. In cases where various customers subject the power system to sudden electrical loads, transients are generated in the system affecting the magnitude and phase of the supplied power signal.
Since the storage of data in a computer system is predicated on the continuous operation of the computer, power interruptions can adversely affect the integrity of the stored data. When the power drops abruptly, the computer will terminate operation probably with a high possibility of component damage. In order to overcome this situation, computers operating in a data processing system environment have been supplied with uninterruptible power supplies. Uninterruptible power supplies have been designed where the primary commercial power source and the reserve power source are connected in parallel. Both the primary power source and the reserve power source are continuously operated and both sources contribute to the energization of the load. This is a completely redundant system, and should either power source fail, the results are not apparent to the load which is continuously energized. Such an uninterruptible power supply system typically uses a ferroresonant transformer with two input primaries which are coupled to energize a single secondary. Through the use of properly designed high reluctant shunts, the two power sources do not transmit power to each other. Both power sources cooperate to share the load's power needs. The disadvantage of this particular arrangement is the expensive transformer design of a ferroresonant transformer having carefully designed high reluctant shunts and symmetrical construction to permit the two power sources to share the load.
An uninterruptible power supply arrangement which permits the use of a less expensive power coupling arrangement has been developed. The power supply includes a commercially A. C. line power source and a D.C. voltage energized inverter power source connected in parallel to a switching mechanism which alternately couples one or the other of the two power supplies to a load to be energized. This power supply design advantageously eliminates the need for an expensive ferroresonant transformer and provides redundancy to provide a substantially uninterruptible power to the load. However, the need for switching devices reduces the reliability of the circuit. In addition, the auxiliary power source comprising the inverter must be synchronized in frequency with the A. C. power line signal which requires complicated synchronizing circuitry. The switching action must be sufficiently fast to handle the transition of a load from a failed A. C. power line to the reserve power source or inverter circuit without inducing damaging transient signals into the circuit. The switching must disconnect the failed power source so it does not become a load for the active power source.
It is therefore a principal object of this invention to provide a switching circuit for use in an uninterruptible power supply which will switch at a high frequency rate.
It is another object of this invention to provide a switching circuit which will switch an A. C. power signal in phase with the line A. C. power signal within one-half cycle of operation.
It is a further object of this invention to provide a switching circuit for use in an uninterruptible power circuit which is simple in construction and low in cost.